Landfill gases typically consist of 30-70 mole percent methane in carbon dioxide. They also contain a host of trace hydrocarbon impurities along with water and hydrogen sulfide. These trace impurities need to be removed prior to the bulk separation of methane from carbon dioxide, since these impurities may adversely affect the separation capacity of the selective adsorbent as well as irreversibly destroying the adsorbent employed in the PSA system.
Bingham, U.S. Pat. No. 4,000,990 describes a system comprised of a thermal swing adsorption (TSA) section followed by a pressure swing adsorption (PSA) section for purifying a fluid stream, stated to be applicable in the purification of landfill gas to recover methane. In the TSA pretreat section water and trace amounts of heavier hydrocarbons are removed by adsorption at a superatmospheric pressure and the non-adsorbed effluent is sent to the PSA section wherein CO.sub.2 is adsorbed and unadsorbed methane is recovered as product. The TSA section of the patent arrangement is regenerated by passing a heated fluid stream (of void gases) through the adsorbent bed in a closed loop recycle regeneration scheme. The purge effluent is cooled to remove part of the condensible impurities, reheated and recirculated. In the subsequent cooling of the regenerated pretreat column a part of the circulating cool gas is vented. After the bed has been cooled it is repressured to the adsorption pressure with the pretreated gas effluent discharged from a companion pretreater column.